Yup, as I said up there ^^ it's because you have PHP notices being injected into the JSON response to the AJAX calls ...
http://screencast.com/t/TX7oMy9ht
In this case from a Jumi plugin. As you can see, the AJAX response with the JSON event data in it has exactly the same set of PHP notices in it as the page itself does on load.
Having those PHP logging msgs in the JSON response causes the Javascript JSON decoding to blow up with "Unexpected token <", because that's HTML, not JSON.
This is problematic for us, because we are AJAX / JSON (so technically AJAJ) heavy, and anything which tosses a PHP warning, error or notice will cause this problem in JSON responses, if error logging is enabled. Not just from our code, but anything in J! or any 3rd party system plugin (as in this case).
One of these days we might attempt to work round this. Convert all our Request.json() calls (which automatically parse the response, so nothing we can do to prevent the error) to plain Request(), which doesn't automatically JSON decode, then strip out any HTML from before the JSON, before manually JSON.decode()'ing it. Also probably don't do that if debug mode is enabled in J! or Fabrik, otherwise it would make debugging a lot harder.
But that's a lot of work.
-- hugh
